Antennas for wellbore logging tools and methods of manufacture

ABSTRACT

An antenna assembly includes a bobbin positionable about an outer surface of a tool mandrel and providing an outer bobbin surface. A coil is wrapped about and extends longitudinally along at least a portion of the outer bobbin surface. An inner sleeve interposes the bobbin and the tool mandrel and receives a plurality of ferrites that interpose the tool mandrel and the coil.

BACKGROUND

During drilling operations for the extraction of hydrocarbons, a varietyof recording and transmission techniques are used to provide or recordreal-time data from the vicinity of a drill bit. Measurements of thesurrounding subterranean formations may be made throughout drillingoperations using downhole measurement and logging tools, such asmeasurement-while-drilling (MWD) and/or logging-while-drilling (LWD)tools, which help characterize the formations and aid in makingoperational decisions. Wellbore logging tools make measurements that maybe used to determine the electrical resistivity (or its inverse,conductivity) of the formations being penetrated, where the electricalresistivity indicates various features of the formations. Thosemeasurements may be taken using one or more antennas coupled to orotherwise associated with the wellbore logging tools.

Logging tool antennas are often formed by positioning coil windingsabout an axial section of the logging tool, such as a drill collar. Aferrite material or “ferrites” are sometimes positioned beneath the coilwindings to increase the efficiency and/or sensitivity of the antenna.The ferrites facilitate a higher magnetic permeability path (i.e., aflux conduit) for the magnetic field generated by the coil windings, andhelp shield the coil windings from the drill collar and associatedlosses (e.g., eddy currents generated on the drill collar).

BRIEF DESCRIPTION OF THE DRAWINGS

The following figures are included to illustrate certain aspects of thepresent disclosure, and should not be viewed as exclusive embodiments.The subject matter disclosed is capable of considerable modifications,alterations, combinations, and equivalents in form and function, withoutdeparting from the scope of this disclosure.

FIG. 1 is a schematic diagram of an exemplary drilling system that mayemploy the principles of the present disclosure.

FIG. 2 is a schematic diagram of an exemplary wireline system that mayemploy the principles of the present disclosure.

FIG. 3A is a partial isometric view of an exemplary wellbore loggingtool.

FIG. 3B is a schematic side view of the wellbore logging tool of FIG. 3A

FIG. 4 is a cross-sectional side view of an exemplary antenna.

FIG. 5A is a cross-sectional side views of another exemplary antenna.

FIG. 5B is an isometric view of a portion of the antenna assembly ofFIG. 5A.

DETAILED DESCRIPTION

The present disclosure relates generally to wellbore logging tools usedin the oil and gas industry and, more particularly, to antennas used inwellbore logging tools and methods of manufacturing the antennas.

The embodiments described herein reduce the complexity in assemblingantennas used in wellbore logging tools while also providing mechanicalintegrity to the antenna. In manufacturing or building one of theantennas described herein, a plurality of ferrites may be positionedabout an outer surface of a tool mandrel and extend circumferentiallyabout the outer surface. In some cases, a groove is defined in the outersurface of the tool mandrel and the plurality of ferrites are securedwithin the groove by being molded into the groove or alternativelyprinted into the groove using an additive manufacturing process. Inother cases, the plurality of ferrites may be seated within a pluralityof channels defined on the inner surface of an inner sleeve thatinterposes a bobbin and the tool mandrel. The bobbin may then bepositioned about the outer surface of the mandrel such that theplurality of ferrites interposes the tool mandrel and a portion of thebobbin. In some cases, the bobbin may be molded directly onto the outersurface of the inner sleeve. In other cases, however, the bobbin may beprinted directly onto the outer surface via an additive manufacturingprocess. A coil winding may then be wrapped about the outer surface ofthe bobbin and extend axially along at least a portion of the outersurface.

FIG. 1 is a schematic diagram of an exemplary drilling system 100 thatmay employ the principles of the present disclosure, according to one ormore embodiments. As illustrated, the drilling system 100 may include adrilling platform 102 positioned at the surface and a wellbore 104 thatextends from the drilling platform 102 into one or more subterraneanformations 106. In other embodiments, such as in an offshore drillingoperation, a volume of water may separate the drilling platform 102 andthe wellbore 104.

The drilling system 100 may include a derrick 108 supported by thedrilling platform 102 and having a traveling block 110 for raising andlowering a drill string 112. A kelly 114 may support the drill string112 as it is lowered through a rotary table 116. A drill bit 118 may becoupled to the drill string 112 and driven by a downhole motor and/or byrotation of the drill string 112 by the rotary table 116. As the drillbit 118 rotates, it creates the wellbore 104, which penetrates thesubterranean formations 106. A pump 120 may circulate drilling fluidthrough a feed pipe 122 and the kelly 114, downhole through the interiorof drill string 112, through orifices in the drill bit 118, back to thesurface via the annulus defined around drill string 112, and into aretention pit 124. The drilling fluid cools the drill bit 118 duringoperation and transports cuttings from the wellbore 104 into theretention pit 124.

The drilling system 100 may further include a bottom hole assembly (BHA)coupled to the drill string 112 near the drill bit 118. The BHA maycomprise various downhole measurement tools such as, but not limited to,measurement-while-drilling (MWD) and logging-while-drilling (LWD) tools,which may be configured to take downhole measurements of drillingconditions. The MWD and LWD tools may include at least one wellborelogging tool 126, which may comprise one or more antennas capable ofreceiving and/or transmitting one or more electromagnetic (EM) signalsthat are axially spaced along the length of the wellbore logging tool126. As will be described in detail below, the wellbore logging tool 126may further comprise a plurality of ferrites used to shield the EMsignals and thereby increase the azimuthal sensitivity of the wellborelogging tool 126.

As the drill bit 118 extends the wellbore 104 through the formations106, the wellbore logging tool 126 may continuously or intermittentlycollect azimuthally-sensitive measurements relating to the resistivityof the formations 106, i.e., how strongly the formations 106 opposes aflow of electric current. The wellbore logging tool 126 and othersensors of the MWD and LWD tools may be communicably coupled to atelemetry module 128 used to transfer measurements and signals from theBHA to a surface receiver (not shown) and/or to receive commands fromthe surface receiver. The telemetry module 128 may encompass any knownmeans of downhole communication including, but not limited to, a mudpulse telemetry system, an acoustic telemetry system, a wiredcommunications system, a wireless communications system, or anycombination thereof. In certain embodiments, some or all of themeasurements taken at the wellbore logging tool 126 may also be storedwithin the wellbore logging tool 126 or the telemetry module 128 forlater retrieval at the surface upon retracting the drill string 112.

At various times during the drilling process, the drill string 112 maybe removed from the wellbore 104, as shown in FIG. 2, to conductmeasurement/logging operations. More particularly, FIG. 2 depicts aschematic diagram of an exemplary wireline system 200 that may employthe principles of the present disclosure, according to one or moreembodiments. Like numerals used in FIGS. 1 and 2 refer to the samecomponents or elements and, therefore, may not be described again indetail. As illustrated, the wireline system 200 may include a wirelineinstrument sonde 202 that may be suspended into the wellbore 104 by acable 204. The wireline instrument sonde 202 may include the wellborelogging tool 126 described above, which may be communicably coupled tothe cable 204. The cable 204 may include conductors for transportingpower to the wireline instrument sonde 202 and also facilitatecommunication between the surface and the wireline instrument sonde 202.A logging facility 206, shown in FIG. 2 as a truck, may collectmeasurements from the wellbore logging tool 126, and may includecomputing and data acquisition systems 208 for controlling, processing,storing, and/or visualizing the measurements gathered by the wellborelogging tool 126. The computing facilities 208 may be communicablycoupled to the wellbore logging tool 126 by way of the cable 204.

FIG. 3A is a partial isometric view of an exemplary wellbore loggingtool 300, according to one or more embodiments. The logging tool 300 maybe the same as or similar to the wellbore logging tool 126 of FIGS. 1and 2 and, therefore, may be used in the drilling or wireline systems100, 200 depicted therein. The wellbore logging tool 300 is depicted asincluding an antenna assembly 302 that can be positioned about a toolmandrel 304, such as a drill collar or the like. The antenna assembly302 may include a bobbin 306 and a coil 308 wrapped about the bobbin 306and extending axially by virtue of winding along at least a portion ofan outer surface of the bobbin 306.

The bobbin 306 may structurally comprise a high temperature plastic, athermoplastic, a polymer (e.g., polyimide), a ceramic, or an epoxymaterial, but could alternatively be made of a variety of othernon-magnetic, electrically insulating/non-conductive materials. Thebobbin 306 can be fabricated, for example, by additive manufacturing(i.e., 3D printing), molding, injection molding, machining, or otherknown manufacturing processes.

The coil 308 can include any number of consecutive “turns” (i.e.windings of the coil 308) about the bobbin 306, but typically willinclude at least a plurality (i.e. two or more) consecutive full turns,with each full turn extending 360 degrees about the bobbin 306. In someembodiments, a pathway for receiving the coil 308 may be formed alongthe outer surface of the bobbin 306. For example, one or more groovesmay be defined in the outer surface of the bobbin 306 to receive andseat the coil 308. In other embodiments, however, the outer surface ofthe bobbin 306 may be smooth or even. The coil 308 can be concentric oreccentric relative to a central axis 310 of the tool mandrel 304.

As illustrated, the turns or windings of the coil 308 extend about thebobbin 306 at an angle 312 offset from the central axis 310. As aresult, the antenna assembly 302 may be characterized and otherwisereferred to as a “tilted coil” or “directional” antenna. In theillustrated embodiment, the angle 312 is 45°, by way of example, andcould alternatively be any angle offset from the central axis 310,without departing from the scope of the disclosure.

FIG. 3B is a schematic side view of the wellbore logging tool 300 ofFIG. 3A. When current is passed through the coil 308 of the antennaassembly 302, a dipole magnetic field 314 may be generated that extendsradially outward from the antenna assembly 302 orthogonal to the windingdirection. As a result, the antenna assembly 302 may exhibit a magneticfield angle 316 with respect to the tool mandrel 304 and, since theangle 312 (FIG. 3A) is 45°, the resulting magnetic field angle 316 willalso be 45° offset from the central axis 310. As will be appreciated,however, the magnetic field angle 316 may be varied by adjusting ormanipulating the angle 312.

FIG. 4 is a cross-sectional side view of an exemplary antenna assembly400, according to one or more embodiments. The antenna assembly 400 maybe the same as or similar to the antenna assembly 302 of FIGS. 3A and 3Band therefore may be best understood with reference thereto, where likenumerals represent like elements or components that may not be describedagain in detail. In the illustrated embodiment, the tool mandrel 304 maycomprise a generally cylindrical structure that provides an interior 402and an outer surface 404. In some embodiments, the tool mandrel 304 mayfurther provide and otherwise define a saddle 406 that extends along aportion of the outer surface 404 of the tool mandrel 304. The saddle 406may comprise a portion of the outer surface 404 that exhibits areduced-diameter as compared to the remaining portions of the outersurface 404. In the illustrated embodiment, some or all of thecomponents of the antenna assembly 400 may be positioned within thesaddle 406 adjacent the outer surface 404.

The antenna assembly 400 may further provide an outer sleeve 408 thatencapsulates and otherwise houses the various components of the antennaassembly 400 within the saddle 406. More particularly, the outer sleeve408 may exhibit an inner diameter that is greater than an outer diameterof the tool mandrel 304 and a length sufficient to extend axially acrossthe saddle 406. Moreover, the outer sleeve 408 provides acircumferential encapsulation by extending about the central axis 310 ofthe tool mandrel 304. The upper and lower ends of the outer sleeve 408may be coupled to the outer surface 404 of the tool mandrel 304 ateither end of the saddle 406 via one or more mechanical fasteners 410such as, but not limited to, snap rings, latches, bolts, screws, orother known mechanical fasteners.

The outer sleeve 408 can be formed of a nonconductive or nonmetallicmaterial such as, but not limited to, fiberglass, a polymer or polymericmaterial (e.g., polyether ether ketone or PEEK), a nickel-based alloy, achromium-based alloy, a copper-based alloy, INCONEL®, MONEL®, anadvanced composite, and/or any combination thereof. As will beappreciated, different materials or combinations of materials can beprovided in multiple layers to form the outer sleeve 408, withoutdeparting from the scope of the disclosure.

The antenna assembly 400 may further provide at least one groove 412defined in the outer surface 404 of the tool mandrel 304. The at leastone groove 412 may be configured to receive and seat a plurality offerrites 414. In some embodiments, as illustrated, one or more dividers416 may be provided within the groove 412 and may otherwise extendradially outward from the groove 412. The dividers 416 may serve toprevent physical contact between laterally adjacent ferrites 414 thatmay be seated within the groove 412, and thereby prevent a continuousmagnetic path between the laterally adjacent ferrites 414. The dividers416 may be made of a variety of materials including, but not limited to,a high temperature plastic, a thermoplastic, a polymer (i.e.,polyimide), a ceramic, an epoxy material, or any combination thereof. Inat least one embodiment, the dividers 416 may be machined into the outersurface 404 of the tool mandrel 304 when the groove 412 is defined. Insuch embodiments, the groove 412 may comprise a plurality of groovesthat are separated by the dividers 416.

In general, the dividers 416 may exhibit a relative permeability (μ_(r))of approximately 1, which is equivalent to the permeability of freespace or air (μ_(o)). Accordingly, the dividers 416 may be consideredsubstantially equivalent to providing air gaps between the adjacentferrites 414, often called “air-gapping,” which essentially serves as anon-magnetic insulator between the adjacent ferrites 414.

The bobbin 306 may be positioned within the saddle 406 radially outwardfrom the ferrites 414 such that the ferrites 414 radially interpose thebobbin 306 and the tool mandrel 304. The bobbin 306 may provide andotherwise define an inner bobbin surface 418 a and an outer bobbinsurface 418 b opposite the inner bobbin surface 418 a. The inner bobbinsurface 418 a may be smooth and otherwise even, but may alternativelyprovide a variable inner surface, such as by defining one or morechannels, grooves, etc. The coil 308 may be wrapped about the outerbobbin surface 418 b and extend (i.e., wind) axially along at least aportion thereof. In some embodiments, similar to the inner bobbinsurface 418 a, the outer bobbin surface 418 b may be smooth or even. Inother embodiments, however, the outer bobbin surface 418 b may define aplurality of winding grooves 420 configured to receive and seat theseveral turns of the coil 308.

In some embodiments, a protective layer 422 may be formed about thebobbin 306 within the saddle 406. The protective layer 422 may beconfigured to secure the bobbin 306 within the saddle 406 whilesimultaneously permitting propagation of signals from the antennaassembly 302 (FIGS. 3A-3B). The material of the protective layer 422 canbe any material that is capable of withstanding downhole conditions,such as elevated temperatures and pressures, and also capable ofwithstanding exposure to common wellbore fluids, such as drillingfluids, contaminants, oil, gas, etc. The protective layer 422 can beformed of a nonconductive and/or nonmetallic material, such as a rubbermaterial, a polymer, or and/or a polymeric material. In at least oneembodiment, the protective layer 422 is made of a fluoropolymerelastomer, such as VITON®.

The ferrites 414 may be made of any ferritic or ferromagnetic materialthat has a relative magnetic permeability greater than 100, such as ironor an iron-based alloy. In some embodiments, the ferrites 414 may beformed of any soft magnetic material, such as manganese zinc (MnZn). Theferrites 414 may be positioned to radially interpose the coil 308 andthe underlying tool mandrel 304 and thereby shield the coil 308 fromeddy currents that may be generated by the tool mandrel 304 duringdownhole operation. As will be appreciated, this may increase theazimuthal sensitivity and/or increase the efficiency/field strength ofthe antenna assembly 400.

The ferrites 414 may be manufactured via a variety of processes. In someembodiments, for instance, the ferrites 414 may be machined out of asolid block of material. In such embodiments, for example, the block ofmaterial may comprise an iron powder or a ferrite powder that is pressedto form the block of material, and the ferrites 414 are machined out ofthe block of material to desired dimensions and/or geometry. In at leastone embodiment, the block of material used to provide the ferrites 414may comprise FLUXTROL® 100, available from Fluxtrol, Inc. of AuburnHills, Mich., USA.

In other embodiments, the ferrites 414 may be molded from a mixture of apowder iron/ferrite material and a binder. The binder may include asilicone-based rubber, an elastomer, an RTV, a polymer (e.g., apolyimide), a ceramic, or an epoxy. The mixture is then pressed into amold that corresponds to the specific dimensions and intricate geometryof the given ferrite 414 being manufactured. Upon cooling, the ferrite414 may then be removed for placement in the antenna assembly 400.

In yet other embodiments, the ferrites 414 may be printed via anadditive manufacturing (e.g., 3D printing) process. Suitable additivemanufacturing processes that may be used to print the ferrites 414include, but are not limited to, laser sintering (LS) [e.g., selectivelaser sintering (SLS), direct metal laser sintering (DMLS)], lasermelting (LM) [e.g., selective laser melting (SLM), lasercusing],electron-beam melting (EBM), laser metal deposition [e.g., direct metaldeposition (DMD), laser engineered net shaping (LENS), directed lightfabrication (DLF), direct laser deposition (DLD), direct laserfabrication (DLF), laser rapid forming (LRF), laser melting deposition(LMD)], any combination thereof, and the like. In at least oneembodiment, the additive manufacturing technique may employ fusiondeposition modeling (FDM) technology.

Once manufactured to desired dimensions and geometry, the ferrites 414may be positioned within the groove 412 and otherwise secured to thetool mandrel 304. In some embodiments, for example, the ferrites 414 maybe secured within the groove 412 using an industrial adhesive or glue,such as an epoxy or RTV silicone. In other embodiments, the ferrites 414may be secured within the groove 412 using one or more mechanicalfasteners, such as screws, bolts, pins, snap rings, etc., withoutdeparting from the scope of the disclosure.

In some embodiments, the ferrites 414 may be positioned in the groove412 and otherwise secured to the tool mandrel 304 during manufacture ofthe ferrites 414. More particularly, in at least one embodiment, theferrites 414 may be molded directly into the groove 412 duringmanufacture.

In other embodiments, the ferrites 414 may be printed directly into thegroove 412 via additive manufacturing. In such embodiments, the ferrites414 may be printed by rotating the tool mandrel 304 about the centralaxis 310 while progressively building the ferrites 414 with a 3Dprinting machine (not shown) to desired dimensions. Alternatively, the3D printing machine may be configured to move (i.e., rotate) about thecentral axis 310 of the tool mandrel 304 to progressively build up theferrites 414 to desired dimensions within the groove 412. In yet otherembodiments, a combination of rotating the tool mandrel 304 and movingthe 3D printing machine about the central axis 310 may be employed toprogressively build up the ferrites 414 to desired dimensions.

As will be appreciated securing the ferrites 414 directly to the toolmandrel 304 (e.g., within the groove 412), whether by manufacturing theferrites 414 individually and subsequently securing them to the toolmandrel 304, or during the manufacture process itself, may help enhancethe mechanical strength of the antenna assembly 400. This may furtherhelp ease the assembly process of the antenna assembly 400. Moreover,this may also simplify and reduce the complexity of the antenna buildupby eliminating common antenna components, which can drive costs down butstill maintain high reliability under downhole conditions.

FIG. 5A is a cross-sectional side view of another exemplary antennaassembly 500, according to one or more embodiments. The antenna assembly500 may be similar in some respects to the antenna assembly 400 of FIG.4 and therefore may be best understood with reference thereto, wherelike numerals represent like elements or components that may not bedescribed again in detail.

Similar to the antenna assembly 400, for example, the antenna assembly500 may include the tool mandrel 304 and the reduced-diameter saddle 406may extend along a portion of the outer surface 404 thereof. The outersleeve 408 may extend across the saddle 406 to encapsulate and otherwisehouse the various components of the antenna assembly 500 within thesaddle 406. The antenna assembly 500 may further include the bobbin 306,and the coil 308 may be wrapped about the outer bobbin surface 418 b andextend (i.e., wind) axially along at least a portion thereof. In theillustrated embodiment, the outer bobbin surface 418 b provides thewinding grooves 420 that receive and seat the coil 308, but the windinggrooves 420 could alternatively be omitted from the embodiment. Theprotective layer 422 may also be formed about the bobbin 306 within thesaddle 406, as generally described above.

Unlike the antenna assembly 400 of FIG. 4, however, the antenna assembly500 may include an inner sleeve 502 that receives the ferrites 414 andinterposes the bobbin 306 and the tool mandrel 304. The inner sleeve 502may be made out of any non-magnetic, electrically insulating, and/ornon-conductive material including, but not limited to a non-magneticmetal (e.g., 718 INCONEL®, beryllium copper alloy, such as TOUGHMET®,etc.), a high temperature plastic, a thermoplastic, a polymer (e.g., apolyimide), a ceramic, an epoxy material, a composite material (e.g.,fiberglass), or any combination thereof. The inner sleeve 502 can befabricated, for example, by additive manufacturing (i.e., 3D printing),molding, injection molding, machining, forming, or other knownmanufacturing processes. The inner sleeve 502 may prove advantageous inenhancing the mechanical strength of the antenna assembly 500.

The inner sleeve 502 may provide an inner sleeve surface 504 a and anouter sleeve surface 504 b. The outer sleeve surface 504 b may bepositioned radially inward from the inner bobbin surface 418 a of thebobbin 306. In at least one embodiment, the outer sleeve surface 504 bmay physically engage the inner bobbin surface 418 a, but it is notrequired. Moreover, as illustrated, a plurality of ferrite channels 506may be defined on the inner sleeve surface 504 a of the inner sleeve502. The ferrite channels 506 may be configured to receive and seat theferrites 414, which, as discussed above, radially interpose the coil 308and the underlying tool mandrel 304 to shield the coil 308 from eddycurrents generated by the tool mandrel 304.

FIG. 5B depicts an isometric view of a portion of the antenna assembly500. In some embodiments, the ferrite channels 506 may be defined in theinner sleeve 502 such that they extend generally orthogonal to thewinding grooves 420 defined in the outer bobbin surface 418b; i.e., atan angle rotated 90° from the angle 312 (FIG. 3A) offset from thecentral axis 310 (FIG. 3A). Accordingly, the ferrites 414 may becharacterized as “tilted” ferrites, as they are required to be tiltedabout the curvature of the inner sleeve 502. In some embodiments, eachferrite channel 506 may be configured to receive a single ferrite 414.In other embodiments, however, each ferrite channel 506 may beconfigured to receive two or more ferrites 414 arranged end-to-end.

In some embodiments, as illustrated, each ferrite channel 506 may beformed and otherwise separated by a ridge or divider 508 defined on theinner sleeve surface 504 a and extending radially inward. When theferrites 414 are received within the ferrite channels 506, the dividers508 may serve to prevent physical contact between laterally adjacentferrites 414, and thereby prevent a continuous magnetic path between thelaterally adjacent ferrites 414. As part of the inner sleeve 502, thedividers 508 may also be made of a non-magnetic material, which exhibitsa relative permeability (μ_(r)) of approximately 1. Accordingly, thedividers 508 may be considered substantially equivalent to providing airgaps between the adjacent ferrites 414, and thereby serving asnon-magnetic insulators between the adjacent ferrites 414.

One or more alignment protrusions 510 may be provided on the innersleeve surface 504 a and otherwise extend radially inward from the innersleeve 502. The alignment protrusion 510 may be configured to mate withan alignment groove or notch (not shown) defined in the outer surface404 of the tool mandrel 304. By mating the alignment protrusion 510 withthe alignment notch, the antenna assembly 500 may be able to be alignedaxially and/or rotationally with axially adjacent (i.e., uphole ordownhole) wellbore logging tools in a downhole assembly.

Moreover, one or more holes 512 may be cooperatively defined between thebobbin 306 and the inner sleeve 502 and used to couple the twocomponents. FIG. 5B depicts one-half of the combination of the bobbin306 and the inner sleeve 502. Alignment pins 514 (one shown) may beinserted into the holes 512 to help axially align the other half of thecombination of the bobbin 306 and the inner sleeve 502. The alignmentpins 514 may also prove useful in preventing relative rotation betweenthe bobbin 306 and the inner sleeve 502 during operation.

Moreover, in some embodiments, the bobbin 306 and the inner sleeve 502may be coupled using one or more anti-rotation devices 516 that radiallyextend at least partially through each of the bobbin 306 and the innersleeve 502. As will be appreciated, the anti-rotation device(s) 516 mayprove advantageous in angularly aligning the bobbin 306 with the innersleeve 502 for desired operation. In other embodiments, however, theanti-rotation device(s) 516 may alternatively comprise a keyway or asimilar mechanism that prevents relative axial and rotational movementbetween the bobbin 306 and the inner sleeve 502, without departing fromthe scope of the disclosure.

The antenna assembly 500 may be manufactured and otherwise built byfirst placing the ferrites 414 within the ferrite channels 506 and thenpositioning the inner sleeve 502 about the tool mandrel 304 (e.g.,within the saddle 406). In some embodiments, the bobbin 306 may besubsequently molded directly onto the outer sleeve surface 504 b. Inother embodiments, the bobbin 306 may be subsequently printed directlyonto the outer sleeve surface 504 b via any of the additivemanufacturing processes mentioned herein. In such embodiments, thebobbin 306 may be printed by rotating the tool mandrel 304 about thecentral axis 310 (FIG. 5A) while progressively building the bobbin 306with a 3D printing machine (not shown) to desired dimensions.Alternatively, the 3D printing machine may be configured to move aboutthe central axis 310 to progressively build up bobbin 306 to desireddimensions. In yet other embodiments, a combination of rotating the toolmandrel 304 and the 3D printing machine about the central axis 310 maybe employed to progressively build up the bobbin 306 to desireddimensions.

Placing the ferrites 414 in the inner sleeve 502 and positioning thebobbin 306 about the inner sleeve 502 may prove advantageous inproviding flexibility in changing the ferrite 414 design withoutrequiring a commensurate design change for the bobbin 306. Thus, thebobbin 306 and the inner sleeve 502 may comprise interchangeablecomponents of the antenna assembly 500. In such embodiments, a universaldesign for the bobbin 306 may be employed while the design of the innersleeve 502 may be altered to fit varying logging needs and to otherwiseconform to varying ferrite 414 designs. As a result, the design orconfiguration of the coil 308 may be maintained, while the ferrites 414may be adjusted to meet particular logging needs. This mayadvantageously ease the assembly process for the antenna assembly 500.

Embodiments disclosed herein include:

A. Antenna assembly that includes a bobbin positionable about an outersurface of a tool mandrel and providing an outer bobbin surface, a coilwrapped about and extending longitudinally along at least a portion ofthe outer bobbin surface, and an inner sleeve that interposes the bobbinand the tool mandrel and receives a plurality of ferrites that interposethe tool mandrel and the coil.

B. A method of manufacturing an antenna assembly that includespositioning an inner sleeve about an outer surface of a tool mandrel,the inner sleeve having a plurality of ferrites arranged within acorresponding plurality of ferrite channels defined on an inner sleevesurface of the inner sleeve, positioning a bobbin about the tool mandrelwith the inner sleeve interposing the bobbin and the tool mandrel, andwrapping a coil about an outer bobbin surface of the bobbin such thatthe plurality of ferrites interposes the tool mandrel and the coil.

C. A method that includes introducing a wellbore logging tool into awellbore, the wellbore logging tool including a tool mandrel, a bobbinpositioned about an outer surface of the tool mandrel and providing anouter bobbin surface, a coil wrapped about the outer bobbin surface, anda plurality of ferrites coupled to an inner sleeve that interposes thebobbin and the tool mandrel, wherein the plurality of ferrites interposethe tool mandrel and the coil, and obtaining measurements of asurrounding subterranean formation with the wellbore logging tool.

Each of embodiments A, B, and C may have one or more of the followingadditional elements in any combination: Element 1: wherein the pluralityof ferrites are seated within a plurality of ferrite channels defined onan inner sleeve surface of the inner sleeve. Element 2: wherein theplurality of ferrite channels are defined by one or more dividerspositioned between laterally adjacent ferrites of the plurality offerrites. Element 3: wherein the bobbin defines one or more windinggrooves on the outer bobbin surface and the coil is received within theone or more winding grooves, and wherein the plurality of ferritechannels extend generally orthogonal to the one or more winding grooves.Element 4: wherein the inner sleeve comprises a material selected fromthe group consisting of a non-magnetic metal, a plastic, athermoplastic, a polymer, a ceramic, an epoxy material, a compositematerial, and any combination thereof. Element 5: wherein the innersleeve is printed via additive manufacturing. Element 6: furthercomprising one or more anti-rotation devices that couple the bobbin andthe inner sleeve. Element 7: wherein tool mandrel defines areduced-diameter saddle, the antenna assembly further comprising aprotective layer formed about the bobbin and the coil within the saddle,and an outer sleeve that extends axially across the saddle toencapsulate the bobbin and the protective layer. Element 8: wherein thecoil is wound about the outer bobbin surface at an angle offset from acentral axis of the tool mandrel.

Element 9: further comprising printing the plurality of ferrites via anadditive manufacturing process, and securing the plurality of ferriteswithin the plurality of ferrite channels using at least one of anadhesive or one or more mechanical fasteners. Element 10: furthercomprising separating laterally adjacent ferrites of the plurality offerrites with one or more dividers defined in the inner sleeve surface.Element 11: wherein positioning the bobbin about the tool mandrelcomprises molding the bobbin onto an outer sleeve surface of the innersleeve. Element 12: wherein positioning the bobbin about the toolmandrel comprises printing the bobbin onto an outer sleeve surface ofthe inner sleeve via an additive manufacturing process. Element 13:further comprising coupling the bobbin and the inner sleeve with one ormore anti-rotation devices. Element 14: wherein the inner sleeve and thebobbin are arranged in a reduced-diameter saddle defined in the outersurface of the tool mandrel, the method further comprising positioning aprotective layer about the bobbin and the inner sleeve within thesaddle, and encapsulating the inner sleeve, the bobbin, and theprotective layer within the saddle with an outer sleeve that extendsaxially across the saddle. Element 15: further comprising winding thecoil about the outer bobbin surface at an angle offset from a centralaxis of the tool mandrel.

Element 16: wherein the tool mandrel is operatively coupled to a drillstring and introducing the wellbore logging tool into the wellborefurther comprises extending the wellbore logging tool into the wellboreon the drill string, and drilling a portion of the wellbore with a drillbit secured to a distal end of the drill string. Element 17: whereinintroducing the wellbore logging tool into the wellbore furthercomprises extending the wellbore logging tool into the wellbore onwireline as part of a wireline instrument sonde.

By way of non-limiting example, exemplary combinations applicable to A,B, and C include: Element 1 with Element 2; and Element 1 with Element3.

Therefore, the disclosed systems and methods are well adapted to attainthe ends and advantages mentioned as well as those that are inherenttherein. The particular embodiments disclosed above are illustrativeonly, as the teachings of the present disclosure may be modified andpracticed in different but equivalent manners apparent to those skilledin the art having the benefit of the teachings herein. Furthermore, nolimitations are intended to the details of construction or design hereinshown, other than as described in the claims below. It is thereforeevident that the particular illustrative embodiments disclosed above maybe altered, combined, or modified and all such variations are consideredwithin the scope of the present disclosure. The systems and methodsillustratively disclosed herein may suitably be practiced in the absenceof any element that is not specifically disclosed herein and/or anyoptional element disclosed herein. While compositions and methods aredescribed in terms of “comprising,” “containing,” or “including” variouscomponents or steps, the compositions and methods can also “consistessentially of” or “consist of” the various components and steps. Allnumbers and ranges disclosed above may vary by some amount. Whenever anumerical range with a lower limit and an upper limit is disclosed, anynumber and any included range falling within the range is specificallydisclosed. In particular, every range of values (of the form, “fromabout a to about b,” or, equivalently, “from approximately a to b,” or,equivalently, “from approximately a-b”) disclosed herein is to beunderstood to set forth every number and range encompassed within thebroader range of values. Also, the terms in the claims have their plain,ordinary meaning unless otherwise explicitly and clearly defined by thepatentee. Moreover, the indefinite articles “a” or “an,” as used in theclaims, are defined herein to mean one or more than one of the elementsthat it introduces. If there is any conflict in the usages of a word orterm in this specification and one or more patent or other documentsthat may be incorporated herein by reference, the definitions that areconsistent with this specification should be adopted.

As used herein, the phrase “at least one of” preceding a series ofitems, with the terms “and” or “or” to separate any of the items,modifies the list as a whole, rather than each member of the list (i.e.,each item). The phrase “at least one of” allows a meaning that includesat least one of any one of the items, and/or at least one of anycombination of the items, and/or at least one of each of the items. Byway of example, the phrases “at least one of A, B, and C” or “at leastone of A, B, or C” each refer to only A, only B, or only C; anycombination of A, B, and C; and/or at least one of each of A, B, and C.

What is claimed is:
 1. An antenna assembly, comprising: a bobbinpositionable about an outer surface of a tool mandrel and having anouter bobbin surface; a coil including a plurality of windings wrappedabout and extending longitudinally along at least a portion of the outerbobbin surface; and an inner sleeve that interposes the bobbin and thetool mandrel and receives a plurality of ferrites that interpose thetool mandrel and the coil.
 2. The antenna assembly of claim 1, whereinthe plurality of ferrites are seated within a plurality of ferritechannels defined on an inner sleeve surface of the inner sleeve.
 3. Theantenna assembly of claim 2, wherein the plurality of ferrite channelsare defined by one or more dividers positioned between laterallyadjacent ferrites of the plurality of ferrites.
 4. The antenna assemblyof claim 2, wherein the bobbin defines one or more winding grooves onthe outer bobbin surface and the coil is received within the one or morewinding grooves, and wherein the plurality of ferrite channels extendgenerally orthogonal to the one or more winding grooves.
 5. The antennaassembly of claim 1, wherein the inner sleeve comprises a materialselected from the group consisting of a non-magnetic metal, a plastic, athermoplastic, a polymer, a ceramic, an epoxy material, a compositematerial, and any combination thereof.
 6. The antenna assembly of claim1, wherein the inner sleeve is printed via additive manufacturing. 7.The antenna assembly of claim 1, further comprising one or moreanti-rotation devices that couple the bobbin and the inner sleeve. 8.The antenna assembly of claim 1, wherein tool mandrel defines areduced-diameter saddle, the antenna assembly further comprising: aprotective layer formed about the bobbin and the coil within the saddle;and an outer sleeve that extends axially across the saddle toencapsulate the bobbin and the protective layer.
 9. The antenna assemblyof claim 1, wherein the coil is wound about the outer bobbin surface atan angle offset from a central axis of the tool mandrel.
 10. A method ofassembling an antenna assembly, comprising: positioning an inner sleeveabout an outer surface of a tool mandrel, the inner sleeve having aplurality of ferrites arranged within a corresponding plurality offerrite channels defined on an inner sleeve surface of the inner sleeve;positioning a bobbin about the tool mandrel with the inner sleeveinterposing the bobbin and the tool mandrel; and wrapping a coilincluding a plurality of windings about an outer bobbin surface of thebobbin such that the plurality of ferrites interposes the tool mandreland the coil.
 11. The method of claim 10, further comprising: printingthe plurality of ferrites via an additive manufacturing process; andsecuring the plurality of ferrites within the plurality of ferritechannels using at least one of an adhesive or one or more mechanicalfasteners.
 12. The method of claim 10, further comprising separatinglaterally adjacent ferrites of the plurality of ferrites with one ormore dividers defined in the inner sleeve surface.
 13. The method ofclaim 10, wherein positioning the bobbin about the tool mandrelcomprises molding the bobbin onto an outer sleeve surface of the innersleeve.
 14. The method of claim 10, wherein positioning the bobbin aboutthe tool mandrel comprises printing the bobbin onto an outer sleevesurface of the inner sleeve via an additive manufacturing process. 15.The method of claim 10, further comprising coupling the bobbin and theinner sleeve with one or more anti-rotation devices.
 16. The method ofclaim 10, wherein the inner sleeve and the bobbin are arranged in areduced-diameter saddle defined in the outer surface of the toolmandrel, the method further comprising: positioning a protective layerabout the bobbin and the inner sleeve within the saddle; andencapsulating the inner sleeve, the bobbin, and the protective layerwithin the saddle with an outer sleeve that extends axially across thesaddle.
 17. The method of claim 10, further comprising winding the coilabout the outer bobbin surface at an angle offset from a central axis ofthe tool mandrel.
 18. A method, comprising: introducing a wellborelogging tool into a wellbore, the wellbore logging tool including a toolmandrel, a bobbin positioned about an outer surface of the tool mandreland providing an outer bobbin surface, a coil wrapped about the outerbobbin surface, and a plurality of ferrites coupled to an inner sleevethat interposes the bobbin and the tool mandrel, wherein the pluralityof ferrites interpose the tool mandrel and the coil; and obtainingmeasurements of a surrounding subterranean formation with the wellborelogging tool.
 19. The method of claim 18, wherein the tool mandrel isoperatively coupled to a drill string and introducing the wellborelogging tool into the wellbore further comprises: extending the wellborelogging tool into the wellbore on the drill string; and drilling aportion of the wellbore with a drill bit secured to a distal end of thedrill string.
 20. The method of claim 18, wherein introducing thewellbore logging tool into the wellbore further comprises extending thewellbore logging tool into the wellbore on wireline as part of awireline instrument sonde.